Battery Control Systems

 

Battery Management

Battery management systems for Lithium Cells

slave bms
Cell monitor board



A battery management system (BMS) is required on a lithium battery to protect the battery from damage.
This damage can be caused by the following:
  • an over voltage being applied to a cell
  • a cell going to a high temperature
  • a cell being over discharged.
  • charging a cell if the temperature is below 0degC.
A BMS is also required for a Lithium battery
  • to keep the batteries balanced. This is to ensure that all the cells have their maximum charge put into them and can supply maximum power before a cell under voltage is detected.
  • to keep a track of the state of charge (SOC) and the state of health (SOH) of the battery.

The BMS must monitor each cell voltage and its temperature. It must measure the instant current and the accumulated current flowing into or out of the battery. From this information it must be able to calculate power remaining in the battery.

The BMS must have various warning and alarm levels for voltage, current and temperature. When these conditions are detected a signal must be passed to the vehicle to start remedial action.

Other Function

A battery management system for use on a vehicle is also required to do the following
  • Send information to the vehicle.
  • Receive commands and requests from the vehicle
  • Control the battery connection
  • Control the charger
  • Monitor leakage currents
  • Test the contactor and relay integrity
  • Detect error conditions where possible

master bms
Master controller monitors a group of cell monitors


Communications

The communications protocol between the BMS and the vehicle is normally CAN. On some vehicles this is a one way transmission with the vehicle only monitoring battery conditions on others a greater degree of control may be used.

It must be remembered that one of the main requirements of a BMS is the protection of the battery and the final control is to break the connection between the battery and the vehicle. If this happens in some installations this can result in other problems so a thorough safety review must be carried out to decide on what action should be taken in the event of an emergency or fault condition occurring.
To this end it is required to have various warnings and alarms to minimise the likelihood of this occurring.

It is common for the BMS to have a hardwired ignition signal to indicate 'Connect battery' and a hardwired safety signal that if broken causes an instant power disconnect. This safety signal might consist of a loop through crash detectors and emergency off buttons.

The BMS is required to communicate and control the charger. This might be over a CAN interface, RS485 or RS232 serial link or a hardwired PWM (pulse with modulation) signal.

There is also a requirement to monitor the battery for test and maintenance purposes. This might be over an in dependant serial link (RS232 or RS485) or over the common CAN interface. Each has advantages and problems.

Other Considerations

The transport and storage of the working battery system must be taken into consideration.

A Lithium battery is regarded as dangerous goods and must be transported as such.

Lithium cells do self discharge over a period. This can be accelerated if the BMS takes excessive current.
The BMS must have the facility to go into a very low power mode to give a long a shelf life as is possible.
In storage there is a requirement to recharge the cells at regular intervals.
This should probably be once a month depending on the cells and the BMS.

If the cells are allowed to over discharge then they may be permanently damaged.

Provision must be made to allow recharging of a battery which has just gone past its cutoff minimum level.
However a grossly over discharged battery must have specialized treatment to attempt to recover damaged cells. Individual cells may need to be replaced.

Consideration must be given to monitoring battery systems while in store. This can be by wired or wireless links.

Battery Electric Vehicles

Battery operated electric vehicles offer low running costs. Set against this are the high cost of the batteries. The current choice at the moment are Sodium or Lithium based technologies.

Sodium technologies operate at 250-300 degC. This means that they can operate in high temperatures with no problems. The problem is that they must be at these temperatures to operate and a cold battery might take 3 days to reach its operating temperature. Charging is not possible to this temperature is reached and power from the battery is required to maintain this temperature. This means that a self discharge of more than10% per day is required to keep the battery warm. At the present the Sodium batteries are not as reliable as they should be but this may change in the future.

Lithium operate at a maximum of 80 degC. which can cause problems in hot or warm countries. Self discharge rates of 1% are typical. If the cells discharge to a very low level then the cells may be damaged and need replacing. Lithium batteries require to be continually monitored to ensure that they do not over discharge, do not get too hot or that the applied charge/regenerative voltage is not too high. Lithium cells should not be charged if their temperature is below 0decC. For BEV Thundersky cells (TS) from China are good value but their quality control could be better. They can supply 200,400 Ah batteries. Other companies can supply high capacity batteries with better quality control but an increased price. There are two choices for BEV cells TS and other can supply 40 to 400AH cells. 80 of these might be placed in parallel to make a battery with each cell being monitored. Tesla in the USA use thousands of smaller cells in a series parallel formation to produce a high power battery. All the systems have similar problems of getting high current through relatively small connection areas.

Power loss is related to the square of the current. If the current doubles then the power loss is X4.

Eg if the internal resistance of a batter system is 1 ohm and the current is 100Amps then

P=I2R power loss= 100x100 x 1 =10kW

If the current is 1000 amps then power loss = 1000 x 1000 x1 = 1000kW

A typical BEV might consist of 80 TS LF 200aH cells. This could provide 80 x 3.2v = 258 volts typical, 340v maximum, Using the typical voltage this would give 258 v x 200a h 51 kwh.

The maximum practical voltage for a battery system would be 500v Above this voltage and practical difficulties occur.
Assume 500v maximum which equates to 500/4.25 = 118 cells in series. assume 400ah cells are used this give an average power of 118 x 3.2 = 377v. .... 377 x 400 = 150 kwh. Two batteries could be used in series referenced to ground. This could give 300 kwh.

slave bms
Master electronics housing

master isolation bms
Master electronics isolation board